Cyclopropyl anion, conrotatory

The electrocyclic reactions of 3-membered rings, cyclopropyl cation and cyclopropyl anion, may be treated as special cases of the general reaction. Thus the cyclopropyl cation opens to the allyl cation in a disrotatory manner (i.e., allyl cation, n = 0), and the cyclopropyl anion opens thermally to the allyl anion in a conrotatory manner (i.e., allyl anion, m = 1). Heterocyclic systems isoelectronic to cyclopropyl anion, namely oxiranes, thiiranes, and aziridines, have also been shown experimentally and theoretically to open in a conrotatory manner [300]. 

Although the cyclopropyl anion itself does not appear to have been investigated, the isoelectronic aziridines are known to open in the predicted conrotatory sense. Thus heating the cis and trans isomers 39 and 40 in the presence of di-methylacetylenedicarboxylate leads with high stereospecificity, by way of a 1,3-dipolar addition, to the products indicated in Scheme 4. The stereochemistry is reversed, as predicted, for the excited-state process.87... 

Among ions, the opening of a cyclopropyl anion is exemplified by the reactions of the trans and cis aziridines 6.55 and 6.58, which are isoelectronic with the cyclopropyl anion. They open in the conrotatory sense to give the W- and sickle-shaped ylids 6.56 and 6.59, respectively, which are isoelectronic with the corresponding allyl anions. This step is an unfavourable equilibrium, which can be detected by the 1,3-dipolar cycloaddition of the ylids to dimethyl acetylenedicarboxylate, which takes place suprafacially on both components to give the cis and trans dihydropyrroles 6.57 and 6.60. The conrotatory closing of a pentadienyl cation can be followed in the NMR spectra of the ions 6.61 and 6.62, and the disrotatory closing of a pentadienyl anion can be seen in what is probably the oldest known pericyclic reaction, the formation of amarine 6.64 from the anion 6.63. 

The conrotatory ring opening predicted by correlation diagrams based on the Woodward-Hoffmann rules for cyclopropyl anions have their conterpart in the isoelectronic aziridines. The interconversions outlined below were investigated by Huisgen et and photochemical ring opening was shown to follow a... 

The generally slow conrotatory ring-opening reaction of cyclopropyl anions as compared to the fast isomerization of the corresponding allyl anions has been supported by means of MO calculations with the parent anions (Table 31) . 

Table 1 gives the results of several MO calculations for the conrotatory ring-opening of the cyclopropyl anion to give the allyl anion, and of the isomerization of the allyl anion 13). 

Table 1. Relative energies (AAHf, kcal/mol) with respect to the cyclopropyl anion of the transition state of the conrotatory cyclopropyl-allyl anion rearrangement A, the allyl anion B, and the transition state of the allyl anion isomerization C.

It is evident from Table 1 that the activation energy for the allyl anion isomerization is much lower than for the conrotation of the cyclopropyl anion to give the allyl anion. Consequently, in order to verify the predicted conrotatory mode one has to trap the first formed allyl anion before it isomerizes to give the thermodynamically most stable isomer, e.g., in a cycloaddition reaction. Exactly this was possible with 2, 3 and 5. So far, however, a similarly fast reaction has not been found for allyl anions 13). 

These data demonstrate that the forbidden disrotatory ring opening of 15 is much slower than the ring-opening reactions of the cyclopropyl anions 10 and 14 which are not prevented from occurring in the predicted conrotatory mode. This kinetical criterium for the thermal conrotatory cyclopropylallyl anion transformation has been published independently by Ford and coworkers 20). 

The thermal cyclopropyl anion to allyl anion rearrangement is predicted to be conrotatory and the photochemical rearrangement to be disrotatory. ... 

We may further extend the analysis of pericyclic reactions by considering that a single p orbital, denoted by the symbol m, can be a participant in a pericyclic reaction. In this analysis, one lobe of the p orbital makes up the top face of a one-atom n system, while the other lobe makes up the bottom face. The participation of a single p orbital is suprafacial if both cycloaddition processes involve only one of the two lobes of the p orbital, and it is antarafacial if the cycloaddition involves both. We may thus predict that the conrotatory opening of the cyclopropyl anion to an allyl anion (Figure 11.72) should take place via an -F 2 ] pathway. Conversely, the opening of the cation would be a -F 2 ] process, giving the opposite stereochemistry in the product." ... 

Cyclopropyl anion The HOMO of the reactant in ground state is W2 having C2 symmetry, therefore, under thermal conditions cyclization will take place in a conrotatory manner (Figure 2.17). 

FIGURE 2.17 Thermal cyclization of cyclopropyl anion in a conrotatory manner. 

Thermolysis or photolysis of suitably substituted aziridines is a very convenient method for the generation of azomethine ylides. Thermal ring opening of aziridines involves a conrotatory pathway like that of a cyclopropyl anion with which it is isoelectronic. As predicted by the orbital symmetry rules, the ring opening of aziridines follows disrotatory mode under the photochemical conditions. The stereochemical consequences of these two modes of ring opening result in the formation of tra/is-azomethine ylide on thermolysis of cw-dicarboxylic ester and of cis-azomethine ylide on photolysis (Scheme 5.17). The rra j-dicarboxylic acid ester behaves in the expected manner. 

Figure 15.9 defined suprafacial and antarafacial interactions of a lone pair in a p orbital (called an uj component). Using these definitions, predict if the ring-opening of the cyclopropyl anion shown below will occur in a conrotatory or disrotatory fashion. What will be the stereochemistry of the product ... 

Direct observation of the electrocyclic cleavage of the cyclopropyl anion has been reported for one case, (Equation 6.8) the observed conrotatory mode is the predicted thermochemical pathway. An isoelectronic system has provided... 

Fig. 9. Conversion of oyclopropyl species to allyl species. Conrotatory changes are on the left disrotatory changes are on the right. Upper section alternative modes of ring opening and orbital correlation of cyclopropyl species. Middle section symmetry state correlation of radical. Bottom section symmetry state correlation of anion.

It was only after Woodward and Hoffmann in 1965 had predicted a conrotatory mode for the thermal cyclopropyl-allyl anion transformation that a new interest developed in this reaction. By means of the iso-7c-electronic aziridine 310 Huisgen and coworkers succeeded in demonstrating that the thermal recation gave a conrotatory formation of azomethine ylid (311) and that the light-induced reaction resulted in a disrotation to give 312. 

By these simple rules Woodward and Hoffmann predicted a disrota-tory course for the opening of the cyclopropyl cation in its ground state to the corresponding allyl cation, while the thermal opening of cyclopropyl radical and anion to allyl radical and anion is conrotatory. A glance at Fig. 2 clearly shows the reason. Reverse predictions can be made for photochemically induced reactions. 

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